I usually leave it until after or at least until I start to see it rising.
It may depend on how long it takes your insulin and food to affect your blood sugar, but my times are not consistent and if I take the insulin first I could end up going low.
It's usually best to take your insulin before breakfast so it can start working by the time you eat. But it's a good idea to check with your doctor or diabetes educator to make sure that's right for your specific situation.